The separation of aromatic and nonaromatic hydrocarbons (generally referred to as dearomatization) from mixed hydrocarbon feeds has long been recognized as necessary and advantageous for a number of varied reasons. For example, when a BTX fraction (benzene, toluene and xylene) is the aromatic fraction it may be used as a raw material in the manufacture of petrochemicals, or as an additive for gasoline to increase its octane rating. Further, the nonaromatic fraction derived from these mixed feeds have varied uses as fuels, solvents and the like and, therefore, are also highly desirable. Such uses for the aromatic and nonaromatic fractions have resulted in the development of numerous dearomatization processes.
Of particular interest and difficulty is the separation of the complex components present in lube oils, wherein the removal of aromatic-type hydrocarbons is necessary to improve the viscosity index, thermal and oxidation stability, and color of the lube oils. The presence of aromatic-type hydrocarbons in lube oils affects the quality of these oils due to the low viscosity index, poor thermal and oxidation stability, high carbon residue, and poor color of such aromatic-type hydrocarbons. The aromatic-type hydrocarbons present in lube oils differ significantly from the BTX fraction found in light hydrocarbon mixtures used in the production of gasoline and, as a result, present vastly different separation problems.
Various processes have been suggested for the separation of the aromatic and nonaromatic hydrocarbons of a mixed feed wherein the aromatic is a BTX fraction. Typical of these processes is a process employing an extraction column for separation of a BTX fraction wherein a selective solvent, BTX and a reflux stream is introduced to a two step distillation column. BTX is then distilled to remove water and entrained solvent. Similarly, a process has been suggested wherein two distillation columns are employed with the BTX fraction and water being distilled in the second column. In addition, a process using two distillation columns wherein the second column is employed to distill the BTX fraction and other components, has been suggested.
One goal of the prior art has related to processes developing a dearomatization process which lowers the cost of dearomatization of the mixed hydrocarbon feed. This reduction in cost for dearomatization can be achieved by improving the selectivity of the selective solvent and by modification of the separation process scheme. U.S. Pat. No. 3,985,644 mentions one such method for modifying the process scheme and reducing dearomatization costs, i.e., by reducing the use of energy-intensive steps, e.g., distillation.
The dearomatization of lube oils is of particular interest. Dearomatized lubricating oils are, generally speaking, naphthenic- and or paraffinic-type viscous materials having a low rate of viscosity change with change in temperature, i.e., relatively high viscosity index, a high degree of thermal and oxidation stability, low carbon-forming tendency, good color, and high flash points. Lubricating oil feedstocks are generally recovered as distillates or bottoms from the vacuum distillation of crude oils. A crude lube oil fraction contains many different chemical components, e.g., paraffins, naphthenes, aromatics, and the like. In order to obtain refined lubricating oils of relatively good quality and high viscosity index, the practice has been to remove components, such as aromatic and polyaromatic compounds, which tend to lower the viscosity index of the lube oil. The removal of these aromatic components has heretofore been carried out by processes as above-described and processes such as disclosed in U.S. Pat. Nos. 2,079,885; 2,342,205; 3,600,302; 2,773,005; 3,291,728; 3,788,980; and 3,883,420.
In copending application U.S. Ser. No. 164,039, filed June 30, 1980, commonly assigned, a solvvent extraction-solvent decantation process is disclosed wherein solvent purification with mixed hydrocarbon feed or raffinate can be employed.
In copending application U.S. Ser. No. 267,427, filed June 4, 1981, commonly assigned, there is disclosed a process for the separation of aromatic and nonaromatic containing feedstocks by use of a unique extraction-decantation process wherein the extraction solvent is preferably a low molecular weight polyalkylene glycol. The instant process provides for an improved process by use of improved mixed extraction solvents.
The process of this invention is to be distinguished from the above described processes in that the instant process provides an energy balanced extraction-separation process that is more economically advantageous, i.e., energy efficient than the above-described processes.